An ignition device for an internal combustion engine or the like generates high frequency plasma and ignites a mixture of air and fuel. Such an ignition device has a power supply for inputting high frequency electromagnetic wave power to a spark plug. The electromagnetic wave provided from power supply is emitted into plasma formation space and generates plasma. Here, the impedance of a transmission line of the electromagnetic wave power, including the plasma formation, space varies depending on the state of the plasma formation space. If the impedance of the power supply and that of the transmission line (i.e., the load) are not matched, part of electromagnetic wave power will be reflected toward power supply and thus decrease the portion of power transferred to the load.
Therefore, Patent Document 1 discloses an impedance matching device that adjusts the impedance of a transmission line by using a stub. This impedance matching device adjusts the impedance of the transmission line by adjusting the short circuit position in the stub by switching multiple switches.